Pp 50 – 51 & Pp 15 &

Proteins Proteins are polymers of amino acids Each has a unique 3D shape Amino acid sequences vary Proteins are major component of cell parts The provide: support and structural components storage of amino acids Several types of proteins are identified: receptor, contractile, defense, enzymes, structural

Building Blocks: Amino Acids There are 20 different types of amino acids All have this general formular The R group is a variable group

Polar vs. Non-polar Amino Acids Some amino acids are polar while some are non-polar This property causes proteins to fold into varied shapes

Significance of polar & non-polar amino acids polar amino acids non-polar amino acids hydrophilic can make hydrogen bonds found in hydrophilic channels & parts of proteins projecting from membranes found on surface of water- soluble proteins hydrophobic forms van der Waals (hydrophobic interactions) with other hydrophobic amino acids found in proteins in interior of membranes found in interior of water- soluble proteins

Peptide Bonds Proteins are formed by condensation A peptide bond is formed

Four levels of protein structure

Primary Structure Primary structure is a chain of amino acids number & unique sequence of amino acids determine the properties of primary structure each position is occupied by one of 20 different amino acids sequence of amino acids is determined by DNA sequence in genes linked by peptide bonds

Secondary structure formed by interaction between amino and carboxyl i.e. -NH and -C=O groups weak hydrogen bonds are formed between – H & = O there are two types:  -helix and  -sheet α- helix formed / polypeptide coils up e.g. sheep wool β- pleated sheet formed e.g. silk in spider web regular repeated folding of amino acid chain secondary structure is stabilized by hydrogen bonds

Tertiary structure These are globular proteins with irregular conformation tertiary structure is the folding up of the polypeptide chain, secondary structure or alpha helix it gives three dimensional globular shape i.e. shape of active site the structure is stabilized by disulphide bridges, hydrogen, ionic& hydrophobic bonds tertiary structure used as enzymes to catalyze biochemical reactions

Quaternary structure made of several polypeptide subunits joined together they maybe conjugated proteins i.e. proteins which combine with a prosthetic group (non- protein molecules) prosthetic groups includes: metals e.g. iron in haemoglobin, nucleic acids as in ribosomes, carbohydrates as in glycoprotein or lipids as in glycolipids

Denaturation of Proteins change in protein’s usual regular structure due to: High temp Change in pH Addition of organic solvent (alcohol, acetone) These factors break the bonds that stabilize the structure

Protein Functions Function Examples Structure Transport Enzymes Movement Hormones Antibodies storage – collagen/keratin/fibrin – myoglobin/hemoglobin, bind & transport oxygen – lysozyme, speeding up metabolic reactions – actin (and myosin tropomyosin (and troponin) – insulin, regulate blood glucose – immunoglobulin – albumin in egg, casein in milk

Difference between Fibrous and Globular Proteins fibrous proteins are long and narrow strands or sheets whereas globular proteins are rounded, spherical or ball shaped fibrous protein made of repetitive amino-acid sequences whereas globular proteins are made of irregular amino acid sequences fibrous proteins are usually insoluble in water whereas globular proteins usually soluble in water globular protein more sensitive to changes in pH, temperature & salt than fibrous proteins fibrous proteins have structural or support functions (roles) whereas globular proteins have metabolic functions such as catalysis & transport function Examples of fibrous proteins: keratin/fibrin/collagen/actin/myosin/silk protein Examples of globular protein: insulin/immunoglobulin/hemoglobin/amylase

Revision Questions Explain how polar and non-polar amino acids help channel proteins and enzymes carry out their functions. [5] Distinguish between fibrous and globular proteins with reference to one example of each protein type. [6] Explain primary structures and tertiary structures of a protein. [3] Draw the structure of a generalized dipeptide. [4] Draw the basic structure of an amino acid, and label the groups that are used in peptide bond formation. [4] List four functions of membrane proteins. [4] Giving a named example of each, state four functions of proteins. [8]